Electric lamps for illumination during nighttime hours, if not turned on and off manually, are commonly controlled either by photocells which turn lamps on at dusk and off at dawn, or by 24-hour programmable timers which turn lamps first on, then off, at designated times each night.
A first object of this invention is to incorporate within a single light-responsive control circuit the on-at-dusk feature of conventional photoelectric controls and the off-at-a-selectable-time capability of conventional timers, while avoiding the following usually undesirable properties of these two types of controls: Conventional photoelectric controls keep lamps switched on until dawn. Conventional 24-hour programmable timers used for lamp control need periodic resetting of turn-on time in accordance with seasonal changes in the occurrence of dusk.
A second object of this invention is to incorporate within said light-responsive control circuit means whereby the included timekeeping circuit is synchronized each day with the time of day, so that, in the event of failure of the electrical power source, synchronization error resulting from such an outage will not last beyond the night following the termination of the outage. This feature is particularly useful in applications where restoration of correct timing by human intervention is not convenient, as in homes of people away on vacation, or where battery backup is inconvenient and/or expensive.
The timekeeping circuitry within this invention is synchronized with solar time by illumination from the sun. The hour and minute designations of the positions of the switches which constitute the turn-off time controlling means, therefore, refer to solar time.
A portion of any discrepancy between the observed turn-off time according to standard time and the turn-off time set on the turn-off time controlling means of this invention is constant and therefore may be allowed for in setting the desired turn-off time. Much of this constant discrepancy is due to the difference between mean solar time and standard time. Standard time is coincident with mean solar time only at certain geographical locations, generally along standard meridians. Some few minutes of any apparent discrepancy between the turn-off time set on the turn-off time controlling means and the observed turn-off time according to standard time may be due to causes which vary on a daily or seasonal basis. Discrepancy between successive days is due primarily to variations in cloudiness, haze, etc. Seasonal variations are caused by the fact that solar time differs from mean solar time by an amount which varies from zero to almost 16 minutes, depending on the inclination of the earth's axis and the speed of the earth's motion in its orbit around the sun. However, the few minutes of variable discrepancies just enumerated may be an asset. Slight variability in the turn-off times of automatically controlled lamps in unoccupied homes may more nearly duplicate human behavior patterns than timers programmed to switch lamps off at unvarying times each night.